1. Field of the Invention
The present invention relates to a communication control method and communication apparatus employing the method. More particularly, the invention relates to a communication control method for securing communications when an L2 loop occurs, and a communication apparatus employing the method.
2. Description of the Related Art
In networks is known an L2 loop (broadcast storm) which is one of extremely frequent faults which may occur as a result of mal-connections of LAN cables (see, e.g., Japanese Patent Application Laid-open Publication No. 2002-252625). When this L2 loop occurs, the transmission paths are not only subjected to high loads all over the subnets, but also there appears phenomenon called black hole phenomenon by which packets are forwarded to the point where the loop is present.
Following is a description of the mechanism by which such an L2 loop occurs. FIGS. 1A and 1B are diagrams describing the L2 loop occurring mechanism.
As shown in FIG. 1A, terminal 1 broadcasts packets with the originator address of “A”, which is the MAC address of the terminal 1, and then, the packets are forwarded by relay switches SW1, SW2 and SW3 such that the packets reach all the terminals 2 and 3 within a subnet.
At that time, the relay switches SW1, SW2 and SW3 learn “A” which is a packet issuing MAC address at a port receiving the packet. In other words, the switches learn that a terminal having the MAC address “A” is present ahead of this port. This enables each of the relay switches to determine a port to which the packet is forwarded based on the result of this learning when a packet having a destination MAC address “A” is next sent from another terminal.
However, as shown in FIG. 1B, when the cables become looped as a result of mal-connection of the LAN cables at the relay switch SW3 for example, a broadcast packet sent from the terminal 1 loops. Thus, each time the packet sent from the terminal 1 loops at the relay switch SW3, the packet from the terminal 1 is sent from the looping point such that the relay switches SW1, SW2 and SW3 within the subnet erroneously learn the MAC addresses as the terminal 1 of the MAC address “A” being present at the looping point, based on the same principle as that used in the previous description.
As a result, even though another terminal intends to send a packet destined for the terminal 1, the packet disappears into the looping point formed at the relay switch SW3, and therefore a packet cannot normally arrive at the terminal 1, disabling the communication to the terminal 1.
Since apparatuses, personal computers and routers using Windows (OS provided by Microsoft Corp.) often send a broadcast packet, there occurs phenomena that the terminal which has once sent the broadcast packet becomes prevented from performing communications within the subnet.
Although Japanese Patent Application Laid-open Publication No. 2002-252625 discloses a technique for detecting the point where such a loop occurs, there has hitherto been no technique for correcting the addresses which have erroneously been learned after the occurrence of the loop. It has hitherto been difficult to communicate with a terminal lying within the loop subnet unless the loop is fundamentally ceased by disconnecting the loop occurring source through physical works such as removing the cables.
Such a disconnection work has led disadvantageously to high time and human costs due to the need to identify the looping point.